1. Field of the Invention
The present invention relates to an information processing apparatus in which a plurality of processing units that perform data processing is connected by a ring bus, and a control method therefor.
2. Description of the Related Art
A token ring network (TRN) type data driving mode processing system is known, which has adopted a concept of a Token Ring Network (TRN) as one type of a ring-shaped communication network and includes a plurality of processing units (Japanese Patent Application Open-Laid No. 64-23340).
In this system, a processing unit which intends to transmit data firstly acquires a packet referred to as a free token which circulates around a bus. Then, the processing unit, which has acquired the free token, copies a destination identifier and processing data on the token, and forwards the token to the bus to transmit the token to a destination module
On the other hand, a processing unit at a reception side, when a token of which destination is own-unit is received, copies processing data and sets a reception completion flag indicating that reception has been completed to the received token, and then launches again the received token to the ring bus. A transmission source node, if a token in which the above-described reception completion flag is not set has returned, sends again the same token. If a token in which the reception completion flag is set has returned, the transmission source node clears the flag attached to the token.
Through the above-described processing, there is realized a communication system in which, when there is one token, for example, one processing unit uses exclusively the ring bus at one time. Such a communication system is referred to as a token-passing-ring system.
However, in an information processing system in which a plurality of modules that perform data processing is connected by a ring bus, if the token passing system is applied thereto, the token must be sent back to a transmission source regardless of success or failure in reception. In other words, even if the reception has been successful, other processing units cannot use the token in the middle of returning from a processing unit that has received the token to a processing unit that has transmitted the token, and as a result, it is not efficient.
Further, in a system in which a plurality of processing units is connected to the ring bus, it is important not to stagnate packets flowing around the ring bus. This is because, once a stream of data flowing around the ring bus is stopped, a processable processing unit is forcibly shifted to a waiting state.